The urinary tract is the most common source of bacteremia in the elderly. Long-term urethral catheters constitute the greatest risk factor for bacteremia in chronic care facilities. E. coli is the leading cause of catheter-associated bacteremia in the elderly (as well as bacteremia unassociated with catheters). While much is known about factors associated with E. coli colonization of the urinary tract, the mechanisms by which this organism escapes the confines of the urinary tract lumen to enter the renal parenchyma and cause pyelonephritis are obscure. In order to study the mechanisms of E. coli renal invasion, an in vitro model of this process that utilizes human renal epithelial cells in tissue culture will be developed. The ability of E. coli to invade and to pass between or through these cells in comparison to control fecal isolates will be measured using well established assays. Inhibitors of epithelial and bacterial processes will be added to the model to identify those that are involved in renal invasion. Next, E. coli isolated from the urine of patients with various well-defined clinical syndromes will be compared to assess the contribution of renal invasive ability to pathogenicity. Epithelial cells from many individuals of different ages will be used to examine the effect of age on susceptibility to renal invasion. Finally, transposon mutagenesis of a virulent strain followed by in vitro and in vivo tests of invasion will be used to identify the genes involved in E. coli invasion of the kidney. These studies will provide important information on the mechanisms E. coli use to penetrate from the lumen of the urinary collecting system to the tissues of the kidney from where they can progress to the bloodstream and cause sepsis and death. Better comprehension of the process of renal invasion may lead to the development of strategies for prevention and intervention and may improve our understanding of the pathogenesis of invasive infections in general.